The present invention relates generally to solenoid actuators, and more specifically to a flux bypass for a solenoid actuator in a fluid valve system.
Solenoid actuators are short-stroke electromagnetic energy conversion devices that convert electrical energy into linear motion of a ferromagnetic plunger along a solenoid axis. Solenoid actuators are commonly used in a variety of applications, including as valve actuators for fluid systems. Solenoid actuators comprise helical coils wound about a ferromagnetic core separated from the ferromagnetic plunger by a variable air gap. The ferromagnetic plunger is typically biased away from the ferromagnetic core with a spring or similar element. A conventional solenoid actuator produces an electromagnetic attractive force between the ferromagnetic plunger and core that is inversely proportional to the square of the air gap width, i.e.:
                                                        F              =                            ⁢                                                1                  2                                ⁢                                                      B                    g                                                        μ                    0                                                  ⁢                                  S                  g                                                                                                        =                            ⁢                                                1                  2                                ⁢                                  μ                  0                                ⁢                                                                            (                      IN                      )                                        2                                                                              (                                                                                                    l                            Fe                                                                                μ                            r                                                                          +                        g                                            )                                        2                                                  ⁢                                  S                  g                                                                                                        ≈                            ⁢                                                1                  2                                ⁢                                  μ                  0                                ⁢                                                                            (                      IN                      )                                        2                                                        g                    2                                                  ⁢                                  S                  g                                                                                        [                  Equation          ⁢                                          ⁢          1                ]            where Bg is the magnetic flux density in the air gap, μ0=0.4π*10−6 H/m is the magnetic permeability of free space, μr is the relative magnetic permeability of the ferromagnetic core, Sg is the surface area of the air gap, g is the air gap width, lFe is the mean path length for magnetic flux through the ferromagnetic material, I is the coil current, and N is the number of turns of the coil. Some solenoid actuators use conical air gaps for an increase in attractive force by increasing air gap surface area Sg.
The inverse proportionality of the attractive force between the ferromagnetic plunger and core to the square of the air gap width can pose a challenge to fine actuator control. In conventional solenoid actuators, attractive force F is dramatically greater at smaller gap widths than at greater gap widths. Some solenoid actuator systems attempt to hold attractive force constant as a function of air gap by appropriately modulating coil current I based on air gap width (i.e. with I(g) ∝g, such that (IN)2/g2 if substantially fixed).